Sintered ceramic materials are used in a wide variety of electronic and optical components including capacitors, magnetic devices such as transformers and inductors, and optoelectronic devices. As these components become smaller, maintaining compositional integrity becomes increasingly important. This is particularly true with respect to metal-containing constituents which tend to volatilize in the sintering process. Magnetic devices such as transformers and inductors illustrate the problem to which the invention is directed. Such devices are essential elements in a wide variety of circuits requiring energy storage and conversion, impedance matching, filtering, EMI suppression, voltage and current transformation, and resonance. As historically constructed, these devices tended to be bulky, heavy and expensive as compared with other circuit components. Manual operations such as winding conductive wire around magnetic cores dominated production costs.
A new approach to the fabrication of such devices was described in U.S. patent application Ser. No. 07/695653 entitled "Multilayer Monolithic Magnetic Components and Method of Making Same" filed by Grader et al on May 2, 1991, and assigned to applicants' assignee. In the Grader et al approach ceramic powders are mixed with organic binders to form magnetic and insulating (non-magnetic) green ceramic tapes, respectively. A magnetic device is made by forming layers having suitable two-dimensional patterns of magnetic and insulating regions and stacking the layers to form a structure with well-defined magnetic and insulating regions. Conductors are printed on (or inserted into) the insulating regions as needed, and the resulting structure is laminated under low pressure in the range 500-3000 psi at a temperature of 60.degree.-80.degree. C. The laminated structure is fired at a temperature between 800.degree. to 1400.degree. C. to form a co-tired composite structure.
A variation of this approach was described in U.S. patent application Ser. No. 07/818669 entitled "Improved Method For Making Multilayer Magnetic Components" filed by Fleming et al. on Jan. 9, 1992, and assigned to applicants' assignee. In accordance with Fleming et al., cracking and magnetic degradation is reduced by forming green ceramic layers having patterns of magnetic and insulating (non-magnetic) regions separated by regions that are removable during sintering. When the green layers are stacked, layers of removable material are disposed between magnetic regions and insulating regions so as to produce upon sintering a magnetic core within an insulating body wherein the core is substantially completely surrounded by a thin layer of free space. In either approach, the preferred materials for the magnetic layers are metal-containing ferrites such as MnZn ferrites. The insulating (non-magnetic) material can be a compatible insulating ceramic material such as Ni ferrite or alumina.
A difficulty that arises in the fabrication of these devices is the tendency of metal or metal oxide constituents in the magnetic material to volatilize during sintering, thereby degrading the magnetic properties of the sintered material. Such loss of metal or metal oxide will be referred to as "metal loss". The conventional method of minimizing metal loss in ceramics is to fire the parts in the presence of sufficient quantity of the self-same material so that volatilization is inhibited and compensated. Applicants discovered, however, that this conventional method is of little value in fabricating small multilayer magnetic components where a layer of insulating material typically surrounds the magnetic core. This is because external metal vapor typically cannot penetrate the insulating material to reach the magnetic core. Moreover, because these components are typically small (a fraction of a cubic cm), the surface to volume ratio is large, aggravating the rate of metal loss. While it was initially believed that metal loss would be limited because the magnetic cores were housed within hermetic boxes of insulating materials, in reality the insulating materials acted as sinks for the metal and aggravated the loss. Accordingly, there is a need for a new way of minimizing metal loss during the fabrication of multilayer ceramic components.